Background: Adherence to medication is the cornerstone to achieve the best treatment outcome. Pharmacists are healthcare professionals found in pivotal positions to assess asthmatic patient’s adherence to medication. A brief, reliable, and valid measure of patient adherence is useful to enable the pharmacists to deliver that vital service. This study aims to develop a reliable and valid adherence assessment tool for asthmatic patients. Methods: The Adherence to Asthma Medication Questionnaire (AAMQ-13) was developed based on an extensive literature review, followed by applying the Delphi technique, and then it was pilot-tested by 55 patients. The final AAMQ-13 was completed by 213 patients. Psychometric evaluation was assessed including reliability, criterion validity, and construct validity. Results: The AAMQ-13 is a feasible 13 item questionnaire, as it can be completed within an average of two minutes. It has high internal validity (Cronbach’s alpha= 0.87). Criterion-concurrent validity was established by comparing the AAMQ-13 to the Test of the Adherence to Inhaler (TAI) and the pharmacy refill records. Criterion-convergent validity was established by comparing the AAMQ-13 to the Asthma Control Test (ACT) questionnaire and the Positive Health Behaviors Scale (PHBS). Construct validity was established through AAMQ-13 factor analysis which revealed two factors explaining 51.76% of the variance. Conclusion: The AAMQ-13 is a reliable and valid questionnaire with several desirable characteristics as it has a high internal validity, good criterion validity, and strong construct validity. The AAMQ-13 is a suitable questionnaire that can identify non-adherent patients and reveal the reasons behind their non-adherence (intentional or unintentional non-adherence).